Along with miniaturization of electronic equipment, the demand for high energy density of a battery used to drive the electronic equipment is being increased. A lithium ion secondary battery rechargeable at high voltage and high energy density, in particular, is being developed increasingly. The lithium ion secondary battery comprises a cathode and an anode which are capable of occluding and releasing lithium ions, and electrolytic solution containing nonaqueous electrolyte. Artificial graphite created by graphitizing mesophase spherules or cokes from low-crystalline carbon material, such as carbon-resin, and, further, highly graphitized material, such as natural graphite, are used as the anode material. Further, material to be graphitized to such an extent that can satisfy the demand for high-energy density is being desired. It is found that graphitized materials, including natural graphite, have the discharge capacity close to a theoretical value, while on the other hand, they have generally a large irreversible capacity of not less than a few tens of mAh/g resulting from the decomposition of electrolytic solution on the anode in the initial stage of electric charge. This is a major hurdle to realize a high performance lithium ion secondary battery. Particularly when propylene carbonate is used for the electrolytic solution, the electrolytic solution is significantly decomposed on the anode. Due to this, the use of propylene carbonate for the electrolytic solution is largely restricted.
Various proposals have been hitherto made for the purpose of reducing the irreversible capacity resulting from the decomposition of the electrolytic solution. JP Laid-open Patent Publications No. Hei 4-370662 and No. Hei 5-335016 disclose a material comprising graphite particle, used for the anode material, whose surface is coated with organic carbonized material. JP Laid-open Patent Publication No. Hei 10-59703 discloses a method of coating carbonaceous powders with carbonized material of petroleum pitch or coal-tar pitch. JP Laid-open Patent Publication No. Hei 11-204109 discloses a material comprising graphite particle whose surface is coated in the form of carbon layer by the chemical vapor deposition method. In addition to these publications, Fukuzuka et al. discloses a graphite particle surface coating method using an oxidation method and an efficiency improving method using an NF3 plasma process (“Carbon film with its surface modified by using the NF3 plasma and the electrochemical property thereof”, 41st Battery Symposium in Japan of Nov. 2000, 2E12 at P 592-593, published by Committee of Battery Technology, Electrochemical Society of Japan). JP Laid-open Patent Publication No. Hei 11-204109 cited above, in particular, discloses an irreversible capacity reduction effect produced when propylene carbonate is used for the electrolytic solution.
However, the anode materials proposed, for example, by JP Laid-open Patent Publications No. Hei 4-370662, No. Hei 5-335016, and No. Hei 10-59703 cited above require a substantially large amount of carbonized material of not less than 10 weight % for coating the graphite powder. As mentioned therein, when these anode materials are analyzed in an X-ray wide-angle diffraction measurement, two diffraction lines corresponding to multilayered structure of the anode material appear clear. This structure often induces reduction of the discharge capacity so that the original capacity of graphite may not be developed, as described in JP Laid-open Patent Publication No. Hei 9-213328. This publication, JP Laid-open Patent Publication No. Hei 9-213328, discloses an anode material comprising 100 parts by weight graphite particle coated with not more than 12 parts by weight carbonized material, and a producing method thereof. However, the production of such an anode material involves powdering processes, such as pulverizing, requiring cumbersome and complicated powder handlings. Referring further to JP Laid-open Patent Publication No. Hei 11-204109 cited above, the anode material comprises uniformly coated graphite power, having a substantially small specific surface area of not more than 1 m2/g. Generally, the anode having a small specific surface area exhibits poorness in quick recharge/discharge characteristic. It also shows poorness in miscibility with a binder resin, thus inducing the disadvantage that the coatability to copper foil is apt to worsen when producing the electrode. Further, Masaki Yoshio, et al (“Effect of Carbon Coating on Electrochemical Performance of Treated Natural Graphite as Lithium-Ion Battery Anode Material”, Journal of Electrochemical Society, Vol. 147, pp 1245-1250, April 2000) discusses the same coating, presenting the data showing that as an amount of material coated increases, the discharge capacity decreases. From this, this related art also has the same problem as the problem of JP Laid-open Patent Publications No. Hei 4-370662 and No, Hei 5-335016. The surface oxidizing technique is extensively studied with the aim of increasing the discharge capacity, but it involves the problem that the effect is not provided stably. Also, the efficiency improving method using the NF3 plasma process is now at a basic research level.
It is an object of the present invention to provide an anode material for a lithium ion secondary battery that can provide enhanced efficiency without reducing a reversible capacity, can reduce an irreversible capacity, and can be used with propylene carbonate electrolytic solution which is restricted-use electrolytic solution in that the electrolytic solution is decomposed significantly in the initial stage of electric charge.